Frequency control system



March 19, 1935. J, H. HAMMOND, JR

FREQUENCY CONTROL SYSTEM Original Filed Nov. 9, 1929 1 mi .33 4 W 'INVENTOR JOHN HA H MOND JR.

ATTORNEY Patented Mar. 19, 1935 t are FREQUENCY CONTROL SYSTEM John Hays Hammond, Jr., Gloucester, Mass.

Original application November 9, 1929, Serial No.

405,873. Divided and this application September 13, 1930, Serial No. 481,706. Renewed July 9 Claims. (01. 250-2) This invention is a division of my application Serial No. 405,873, filed November 9, 1929 entitled Transmission of related radiant frequencles.

The object of the present invention is the pro- 5 duction of radio frequency energy in the form'of two or more separate radiant frequencies having accuratelyidetermined frecpuencyw differences at all times.

More specifically, it is the object of this invention to provide a means and apparatus for producing and radiating currents of two or more radio frequencies a definite and constant difference apart with respect to the frequency spectrum. v I

These and further objects of my invention will become apparent from the following specification taken in connection with the appended drawing.

It is frequently necessary to radiate energy at various radio frequencies when the difference between the two frequencies is of great'importanc'e, such a use would berequired, for instance, in the control of apparatus by two' radio frequencies. Such a control of the frequency allocation of a plurality of radio transmitters is also desirable in radio broadcasting. The fundamental object of the present invention is to enable each of these requirements to be fulfilled.

When it is desired to generate a current of a frequency which shall at all times have a given frequency difference between it and another frequency, and this latter frequency is subject to slight variations, I provide a generator of a current of a frequency a few hundred cycles difference from the desired difference frequency, and modulate the actual difference frequency by this frequency, I then by mechanically vibrating filter means, operated by the fluctuations in the frequency thus produced control the frequency of the controllable radiant frequency.

Having thus briefly described my invention I will now describe it in detail with the use of the accompanying drawing in which the single figure is a circuit illustrating one wayin which my method for controlling the frequency of one of two frequencies between which it is'desired to maintain a constant frequency difference. 7

Attention is now invited to the drawing which relates to the control of the frequency produced by one transmitting device to maintain it at all times a constant frequency difference from the radiations of a second device. The antenna system 82 is adapted to radiate high frequency energy produced by the transmitter shown generally at 86. Similarly, the antenna 84 is adapted to radiate the energy produced by the transmitter indicated generally at 88. The frequency of the last mentioned transmitter 88 is adapted to be controlled by the condenser 90 which latter is adapted to be automatically varied to main- 5 tain the desired frequency difference at all times as will now be described. The frequency transmitted by the antenna 82 is picked up by the inductance 92 and the frequency radiated by the antenna 84 is picked up by the inductance 94 both of which inductances are included in the plate circuit of the demodulating device 98, the function of which will now be described.

The oscillator 96 with its associated circuits is adapted to produce. a .B. frequency which dilfers from'the desired frequency difference between the two radiations by a small number of cycles. This frequency-is supplied to the grid of the plate demodulator 98, the plate voltage of which is supplied as above stated. The demodulator 98 performs the functions of first demodulation to produce the difference frequency, and second demodulation by heterodyne methods to produce low frequency. This has been found an eficient method of using one tube for double purpose when sufficient radio frequency energy is available. Thus the difference frequency existing in the plate circuit will be modulated by the B frequency produced by the oscillator 96. In the plate circuit of the modulator 98, there is included an output circuit 100 which is provided with a by-pass condenser for high and intermediate frequencies. The whole output is so adjusted as to transmit over a broad band to cover the frequencies it may be called upon to transmit.

The energy existing in the circuit 100 is picked up by the secondary 102 and supplied to the coils 104 which surround vibration type reeds 106. These reeds are adapted to vibrate at frequencies of the normal frequency which will exist in circuit 100 and frequencies slightly above and slightly below that frequency.

A plurality of contacts 108 are adapted to be closed by the Vibration of the reeds 106 and are adapted to short circuit various variations of the resistor 112. Resistors 114 and 116 are so arranged with respect to the source 118 that a varying potential will be impressed upon the armature of the motor 120 in accordance with which the contacts 108 are closed. The motor is connected to the condenser 90 in any appropriate manner as through gearing so that rotation of themotor will serve to vary the frequency produced by the oscillator 88. Thus, for instance. if the central contact 110 is closed by the reed 104, which will vibrate to the normal frequency which will exist in this circuit with respect to the armature 102, a perfect bridge will be formed and no potential from the source 118 will be impressed upon the said armature. If, however, other of the contacts are closed, a potential will be impressed upon the armature of the motor 120 which will cause it to rotate in one direction or the other and thus vary the setting of the condenser to control the.

frequency of the transmitter 88. r

The generator 86 is adapted to produce, and the antenna 82 is adapted to radiate a current of one of two frequencies, the difference between which it is desired to accurately control. A current of the second frequency is produced by the transmitter 88 and is controlled by the condenser 90. Say that the antenna system 82 radiates a current of a frequency of 165,000 cycles and the antenna 84 radiates a current of a frequency of 155,000 cycles, the difference between this frequency is 10,000 cycles which frequency will exist in the plate circuit of. the device 98. In this case the oscillator 96 would be arranged to generate a current of a frequency of 9,500 cycles or a frequency which would differ from the beat frequency by a very low frequency of 500 cycles, from the difference frequency between the main radiations. The circuit 100 would be broadly tuned to pass frequencies of the order of 500 .cycles which frequency would be actually controlled by the difference between the frequency of antenna 82 and antenna 84:. The reeds 106 would be tuned to vibrate at, say, from 460 to 540 cycles, the

f central reed, which is adapted to close contact 110, being adapted to vibrate at. 500 cycles which is normally the frequency which will exist in the reed actuating circuit. When the contact 110 is closed, which indicates that the frequency difference of the two transmitters is exactly correct, there will be a balance of the resistance in the bridge arrangement so that no energy from. the battery 118 will be impressed upon the armature of motor 120., If, however, the difference is incorrect, other of the reeds will be vibrating and other contacts will be closed and the bridge will be thrown out of balance. Then a voltage will be impressed by the battery 118 upon the armature of the motor 120 which will cause it to rotate in a direction to appropriately adjust the frequency of the transmitter 88. When the adjustment of the transmitter 88 has caused it to generate a frequency differing from the frequency transmitted by the transmitter 86 by the proper amount, the frequency of 500 cycles will again exist in the windings 104 and the contact 110 will be closed, which will cause the motor 120 to stoprotating until the frequency difference is again varied, at which time the correction will be" again accomplished in a similar manner.

Having thus described my invention, attention is invited to the fact that various embodiments might be devised differing from those shown and described for the purpose of illustration, and that I am not to be limited by the specific form shown and described, but by the scope of my invention as set forth in the appended claims.

I claim:

1. Apparatus for the production of related radio frequencies which comprises means for produc ing a high continuous frequency, means for producing a s cond high continuous frequency, means for producing a low continuous frequency, means for comparing said last mentioned frequency with the difference of said first mentioned frequencies,

means for controlling the frequency of said second named frequency, and bridge means for controlling the last mentioned meansby the frequency obtained by said comparing.

2. Apparatus for the production of'a radio frequency of a fixed frequency from a given high continuous frequency, which comprises means for producing a high continuous frequency, means for controlling the frequency of said frequency, means for producing a low continuous frequency, means for comparing said last mentioned frequency with the difference between said fixed frequency and controllable frequency, and bridge means for controlling said controllable frequency a by the frequency obtained by said last mentioned comparing, including electro-mechanical means operated by said last mentioned frequency to throw the bridge off balance in order to supply current to said frequency controlling means.

3. In asignalling system wherein a frequency spectrum is divided into a plurality of signalling channels each of which is capable ofjtran smitting signals, means for maintaining the carrier energy of each channel properly allocated. with respect to the carrier energy of each of the other channels, comprising, meansv for producing main frequency carrier energy, means for producing-controllable frequency carrier energy for one of said channels, means for controlling. the frequency of said last named energy, means. for combining a portion of said main frequency carrier energy with a, portion of said controllable frequency carrier energy and producing from said combination heterodyne frequency energy, means for producing auxiliary fre quency energy, means for combining said auxil iary frequency energy with said heterodyne frequency energy and producing therefrom control ling energy, and means controlled by'said controlling energy for controlling. said controllable frequency carrier energy frequency controlling.

means. v

4. In a system of the kind described a source of alternating current, a controllable frequency source of alternating current and an auxiliary source of alternating current, means for mixing energy from said three sources comprising a space discharge device having an input circuit,.

an output circuit and an energyiransference circuit, means for impressing energy from. said first named source and said second named. source.

5. In combination with an'electron tube demodulator circuit, three sources of difierent'frequency energy of which two are coupled to the output circuit of said demodulator circuit and the other is coupled to the input circuit thereof, a utilizing circuit, and ;means for coupling the output of the; demodulator circuit to said utilizing circuit. I 6. The method of producing a plurality of'related frequencies which comprises producing a, continuous frequency, producing a second con tinuous frequency, producing a third frequency, comparing said last mentioned frequency with the difference of two first'mentioned frequencies,

and utilizing the difference thus obtained to control the frequency of one of said first men tioned frequencies.

7. The steps in the method of maintaining two alternating currents the frequency of at least one of which is controllable a constant frequency distance apart, which comprise combining the two alternating currents and producing from said combination a heterodyne frequency, producing an auxiliary frequency, combining said produced heterodyne frequency and said auxiliary frequency and producing therefrom a controlling heterodyne frequency, and controlling the frequency of the controllable alternating current in accordance with the frequency of the controlling heterodyne frequency produced.

8. Apparatus for the production of a radio frequency current differing in frequency by a definite amount from a second radio frequency current, which comprises, a source of radio .frequency current, means for controlling the frequency of said radio frequency current, a second source of radio frequency current, means for producing oscillations of a third frequency by combining, beating and rectifying current from both said radio frequency sources, means for producing a current of a fourth and fixed frequency which differs slightly from the frequency of the third current, means for combining, beating and rectifying said third and fourth currents to produce a fifth current, and means for actuating said frequency controlling means in accordance with the frequency of the fifth current to cause the frequency of said fifth current to assume a definite fixed valve.

9. The steps in a method of maintaining the frequency of two alternating currents substantially a constant frequency distance apart which comprise, combining a part of each of the two alternating currents and producing from said combination a beat note having a frequency value varying in accordance with frequency difference variations between the frequencies of said two alternating currents, and varying the frequency of one of said alternating currents in accordance with the frequency value of said beat note in a 20 

